This invention relates to fasteners for joining sheet materials, and further relates to improved fasteners which provide particularly improved installation and locking action in joining a relatively thick member such as a plastic component to a thinner underlying support structure of sheet metal.
In the assembly or erection of structures in which a relatively thick sheet material such as plastic, rigid insulation, or other similar materials, is joined to a relatively thin sheet material, for example typical thin sheet metal, the fastener conventionally employed has been a typical sheet metal screw in which the length of the threaded shank has been matched to accommodate the thickness of the thicker sheet material. In this conventional joint, preformed holes through each of the sheet materials are aligned and the shank of the sheet metal screw is inserted first through the thicker material and then driven through the sheet metal. The two sheets are thus clamped between the fastener threads engaged in the sheet metal and the typical flanged driving head engaged against the exposed surface of the thicker sheet.
Because the flanged head is spaced from the thinner sheet metal by the thickness of the thicker sheet, there is little reinforcement of the thinner sheet metal at the periphery of the aperture in which typically only one or at most two pitches of threading are engaged. As a consequence of the small number of thread turns in engagement with the underlying sheet metal component, there is a danger that the joint will be stripped if the fastener is over-torqued. Also, since the last thread turn adjacent the collar or shoulder is often incompletely formed, it cannot be relied upon to effect engagement with the sheet material, as the fastener will merely spin and attain no clamping action. As such, while sufficient torque must be applied to attain the desired clamping engagement, it must be excessive, as stripping of the joint can occur.
To overcome the problem of stripping and spinning, it has been proposed to place teeth on the flanged driving head. These teeth will engage the sheet material and increase the driving torque, while preventing the application of excessive torque to the threaded connection between the fastener and the sheet metal component. Also, serrated washers may be employed. A preferred approach is to have teeth on a shoulder immediately adjacent the thread for engaging the sheet metal material. These teeth would control the application of torque and also assume that only threads or thread turns which are fully formed engage the sheet metal component.
Prior to the present invention, however, the provision of teeth on a narrow shoulder has been a difficult and expensive procedure, and not cost justified or cost effective.
These deficiencies are eliminated by the improved fastener and method of manufacture of the present invention, as well as the improved joint attained therewith.
The improved fastener of the invention provides axially extending teeth on a narrow shoulder adjacent the fastener thread which bite into the metal sheet material and are particularly useful in biting into the surface of a thin sheet which is fastened to a thicker sheet material. The teeth extend from a medial collar of the fastener at a shoulder formed on the collar which has a larger diameter than the end of the threaded shank to which the shoulder is joined. The head of the fastener is formed on the opposite end of the medial collar. Most importantly, the teeth are provided by a simple and standard process known as knurling, which is performed readily and without considerable expense
In the preferred embodiment, the threading of the shank is performed prior to the knurling operation and the thread extends entirely to its junction with the shoulder. As a result of the knurling of the collar subsequent to threading of the shank, the cold working of the collar material will produce protuberances or teeth which extend axially from the collar and are disposed in spaced, overlapping relation to at least the first thread turn on the fastener shank. The teeth are formed by cold-flow of the material of the shoulder so that the teeth extend from the outer peripheral edge of the shoulder in order to bite into the thin sheet metal proximate the annular periphery of the aperture in which the threaded shank is engaged in the secured joining of the two sheet materials. The overlap of the teeth with respect to the first thread turn insures that sufficient engagement of the teeth with the sheet metal will occur before the shoulder abuts the sheet metal component.